1. Field of the Invention
The present invention relates to a water-soluble film and a package using the same. More particularly, the present invention relates to a water-soluble film made of a modified polyvinyl alcohol (hereinafter, polyvinyl alcohol may be abbreviated as “PVA”), of which the contents of an N-vinylamide monomer unit, and a carboxyl group and a lactone ring, and the degree of polymerization and the degree of hydrolysis are specified, and which is excellent simultaneously in cold water solubility, biodegradability, chemical resistance, and practical physical properties such as strength and stiffness, and a package using the same.
2. Description of the Prior Art
In recent years, as a method for using various chemicals including agrochemicals, laundry detergents, bleaching agents, toiletry products, industrial chemicals, and the like, there has become employed a method in which each of these chemicals is hermetically packaged on a unit dose basis by a water-soluble film (unit dose package), and it is thrown into water still in the packaged form for use, so that the contents are dissolved or dispersed together with the packaging film in water to be used. The unit dose package has advantages in that a hazardous chemical can be used without direct contact therewith during use, that no metering is necessary for use because a predetermined amount of the contents has already been packaged, that the disposal of the container packaging the contents is unnecessary, and the like.
In the art, PVA films have often been used as such water-soluble films for unit dose package. The PVA film generally has features of tough film, excellent transparency, and good printability. However, the PVA increases in crystallinity with an increase in its degree of hydrolysis. Accordingly, the proportion of the crystalline portion which will not dissolve in cold water increases. Therefore, not a PVA with a high degree of hydrolysis, referred to as a completely hydrolyzed type, but an unmodified partially hydrolyzed PVA has been employed for use as a cold water-soluble film for unit dose package, or the like.
The water-soluble film using an unmodified partially hydrolyzed PVA has features of being readily soluble in cold water and hot water, excellent mechanical strength, and the like. However, in recent years, from the viewpoints of workability, chemical resistance, environmental protection, and the like, there has been a demand for a water-soluble film which simultaneously satisfies a large number of performance requirements such as higher dissolving rate in water, low susceptibility to bag breakage due to an impact, low degree of change in solubility in water with time during storage of the film, and good biodegradability.
A conventional unmodified partially hydrolyzed PVA film has a sufficient cold water solubility at the early stage of production of the film. However, when it is stored for a long time, unfavorably, the cold water solubility decreases presumably because of the gradual growth of the crystals in the meantime. In addition, for the unmodified partially hydrolyzed PVA film, when an alkaline or acidic substance is packaged therein, hydrolysis of acetic acid groups remaining in the partially hydrolyzed PVA occurs during storage. Accordingly, crystallization proceeds, so that the film becomes insoluble. For this reason, the film does not satisfy the necessary performance requirements. Further, for a film using an unmodified partially hydrolyzed PVA, when a chlorine-containing compound such as an agrochemical or a bactericide is packaged therein, and allowed to stand for a long time, the film is colored or hardened. In addition, the film is reduced in water solubility with time to become water insoluble or slightly soluble. Therefore, the chemical agent will not dissolve or disperse in water while being packaged in the film. As a result, unfavorably, the initially intended object cannot be attained.
As the ones solving these problems, various water-soluble films made of a modified PVA have been proposed. As examples of the modified PVA, there are known a carboxyl group-modified PVA (“Application and Market of Water-Soluble Polymer” pp. 266-277, CMC, issued in 1984), a hydrolyzate of a copolymer of an allyl ester and a vinyl ester (Japanese Unexamined Patent Application (Kokai) 62-179550), a PVA containing at least one of an oxyalkylene group, a sulfonic group, and a cationic group (Japanese Unexamined Patent Application (Kokai) 63-168437), a PVA containing a 2-pyrrolidone ring (Japanese Unexamined Patent Application (Kokai) 2-124945, U.S. Pat. No. 5,102,950 and EP0354410), a hydrolyzate of a copolymer of N-vinylacetamides and a vinyl ester (Japanese Unexamined Patent Application (Kokai) 5-59113), a hydrolyzate of a copolymer of a diacetone acrylamide and a vinyl ester (Japanese Unexamined Patent Application (Kokai) 5-17597), a hydrolyzate of a copolymer of an oxyalkylene group-containing vinyl ether and a vinyl ester (Japanese Unexamined Patent Application (Kokai) 11-236419), and the like.
Whereas, as the films for packaging chlorine-containing compounds such as a bactericide, polyethylene oxide-based films and cellulose-based films are known. However, they have a very slow rate of dissolution in water and lack in mechanical physical properties, and are hard and brittle, and hence, unfavorably, they have a very small impact resistance at a low temperature. As the ones solving such a problem, there are known water-soluble films obtained by mixing a PVA and a carboxylate-modified PVA with adjuvants such as polyethylene glycol and propyl gallate (Japanese Unexamined Patent Application (Kokai) 53-24351 and U.S. Pat. No. 4,119,604), a water-soluble film obtained by mixing a sulfonic acid-modified PVA with an adjuvant such as propyl gallate (Japanese Unexamined Patent Application (Kokai) 11-222546), a-water-soluble film for packaging an agrochemical obtained by mixing a sodium 2-acrylamide-2-methylsulfonate-modified PVA with a phenol derivative such as bisphenol A (Japanese Unexamined Patent Application (Kokai) 7-118407), a water-soluble thermoplastic film made of a copolymer of N-vinylacetamide and a vinyl comonomer (Japanese Unexamined Patent Application (Kokai) 5-294358), and the like.
However, the water-soluble film made of a carboxyl group-modified PVA has a deficiency in that the solubility in cold water is impaired upon contact with an acidic substance. Further, the carboxyl group-modified PVA may gel by heat. Still further, for the carboxyl group-modified PVA, by increasing the carboxyl group content thereof, it is possible to enhance the solubility in cold water, whereas the reduction in biodegradability of the modified PVA is incurred. The water-soluble film made of such a carboxyl group-modified PVA with a reduced biodegradability will be discharged into environment after being dissolved in water, and hence the use thereof is not preferable.
The film made of a hydrolyzate of a copolymer of an ally ester and a vinyl ester, a hydrolyzate of a copolymer of an oxyalkylene group-containing ally ether and a vinyl ester, a hydrolyzate of a copolymer of an oxyalkylene group-containing vinyl ether and a vinyl ester, or the like has the following problem. Namely, the copolymerizabilities of an ally ester, an oxyalkylene-containing allyl ether, and an oxyalkylene-containing vinyl ether with a vinyl ester are low. Accordingly, the amount of modification of the PVA is restricted to a low level, and hence the water solubility of the film is insufficient. When the amount of modification of the PVA is raised by increasing each amount of an ally ester, an oxyalkylene group-containing allyl ether, and an oxyalkylene-containing vinyl ether relative to the amount of a vinyl ester for copolymerization, the film produced from each resulting modified PVA is not only reduced in biodegradability but also reduced in degree of polymerization of the modified PVA. Therefore, the film produced from the modified PVA cannot be said to have enough strength and impact resistance to be used for packaging purpose, and thus, unpreferably, it will often undergo bag breakage during use.
As for the film made of a modified PVA in which an ionic group such as a sulfonic group or a cationic group has been introduced, and the film made of a hydrolyzate of a copolymer of a diacetone acrylamide and a vinyl ester, the biodegradabilities of the films tend to decrease with an increase in modification amount. In consequence, a water-soluble film simultaneously satisfying requirements in regard to water solubility, biodegradability, and practical physical properties such as strength cannot be obtained yet.
The film made of a 2-pyrrolidone ring-containing PVA, and the film made of a hydrolyzate of a copolymer of N-vinylacetamides and a vinyl ester have a problem of insufficient cold water solubility because a 2-pyrrolidone ring-containing monomer and N-vinylacetamides are nonionic monomers. Attempts have been made to increase the modification amount, reduce the degree of hydrolysis, reduce the degree of polymerization, and perform other methods, in order to solve the problem associated with the cold water solubility. However, a water-soluble film simultaneously satisfying requirements in regard to water solubility, biodegradability, and practical physical properties such as strength cannot be obtained yet.
The films prepared by mixing a PVA and a modified PVA with adjuvants such as propyl gallate and bisphenol A could not be said to have sufficient performances due to the following fact. Namely, the films discolor in brown, and they are reduced in water solubility when a chlorine-containing substance is packaged therein, and stored for a long period. Whereas, the water-soluble film made of a copolymer of N-vinylacetamide and a vinyl comonomer has a slow rate of dissolution in water, and insufficient mechanical physical properties. For this reason, a water-soluble film simultaneously satisfying the requirements in regard to the water solubility, mechanical physical properties, and chlorine resistance cannot be obtained yet.
It is therefore an object of the present invention to provide a PVA-based water-soluble film which overcomes the foregoing deficiencies, has good cold water solubility and biodegradability, and is excellent in practical physical properties such as strength and stiffness, and will not be reduced in various physical properties with time even when it packages various chemicals such as acidic substances and chlorine-containing compounds.